A playing and/or recording device for a disc shaped carrier is known to adjust the disc rotation speed depending on the nature of the disc. Typically two modes are used, namely Constant Angular Velocity and Constant Linear Velocity mode.
In CAV mode the disc is rotated at a constant rotation speed. This is for example the case for Compact Disc-ROM devices which are of widespread use as computer peripherals. In a start phase after the disc has been inserted in the CD-ROM player and is ready to be read, a leading value output means generates a determined start rotation speed value. This value is output to speed servo means which regulate disc actuating means such to rotate the disc at the start rotation speed. An instantaneous disc rotation speed may be obtained from a frequency signal which is generated by frequency generating means. The frequency generating means may for example be realized by a device which is directly measuring the rotations of a motor shaft in the disc actuating means. The speed servo means receive the frequency signal, compare the determined start rotation speed with the instantaneous disc rotation speed calculated from the frequency signal, and consequently regulate the disc actuating means such that the disc keeps rotating at the determined start rotation speed. Any other speed may now be adjusted by using the leading value output means.
Disc players and/or recording devices typically comprise a pick up which may be moved relative to the rotating disc in order to be positioned for reading and/or recording data at a determined location of the disc. In optical disc player for example the pick up comprises optical means which receive light reflected by the disc and project it on light detecting means. This way an output of the light detecting means is indicative of data scanned by the reflected light. The data may for example be recorded along tracks. The tracks form circles or a spiral having a center substantially located at a center of the disc rotation.
In CLV mode the disc is rotated such that data being read and/or recorded using the pick-up appears to be passing by the pick-up at a constant speed. This means that the disc rotation speed is actually higher when the pick-up accesses data near the center of the disc than when it accessed data near a periphery of the disc. This is for example the case in audio Compact Disc players. The disc rotation speed must be adjusted depending on where on the disc the pick-up is to access data. Typically an output of the pick-up, i.e., an output of the light detecting means is processed using signal processing means and a data frequency signal showing at which frequency data is read by the pick-up is obtained. The data frequency signal is compared to a desired frequency corresponding to a determined linear velocity and a speed servo circuit regulates the disc actuating means in a known manner such that the disk rotations speed remains adapted to have a data frequency signal substantially equal to the desired frequency.
Many players and/or recording devices for disc shaped carriers have the possibility to function in either the CAV or CLV mode. In order to realize this compatibility to CAV and CLV it is known to either have two distinct circuits for CAV and CLV or to modify the speed servo circuit of either one such that it may perform in both modes. The latter solution requires a relative high degree of complexity in the modified speed circuit. The former solution requires that in addition to the two distinct circuits, a possibility to switch between both circuits when appropriate be included.